1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit, and more specifically, to an automatic color control (ACC) circuit in a color signal processing circuit.
2. Description of the Background Art
An AGC circuit automatically controls a gain of an amplifier such that the output remains stable while the input may vary. An analog AGC circuit generally is provided with an analog variable gain amplifier and an analog detector for detecting the output of the amplifier to control the gain of the amplifier. When the output level is higher than the reference level, the gain is reduced by the detector. On the other hand, when the output level is lower than the reference level, the gain is increased by the detector. Thus, a stable output is obtained at all times.
One of such AGC circuits is an ACC circuit. The ACC circuit is used in a color signal processing circuit which processes a color signal from a Y/C (luminance/color) separating circuit, and amplifies the color signal so that the amplitude of a burst signal included in the color signal remains stable at all times. An analog ACC circuit generally is provided with an analog variable gain amplifier for amplifying a color signal, and an analog detector for detecting a burst signal included in the amplified color signal to control the gain of the amplifier. When the level of the burst signal is lower than the reference level, the gain is increased by the detector, whereas when the level of the burst signal is higher than the reference level, the gain is reduced by the detector. Therefore, the amplifier amplifies the color signal such that the amplitude of the burst signal included in the amplified color signal remains stable at all times.
Since the performance characteristics of the variable gain amplifier and the detector in the above-described analog AGC circuit (including the ACC circuit) vary depending on the ambient temperature and the power supply voltage, there was the problem that the output of the AGC circuit varied accordingly.
An object of the present invention is to provide an automatic gain control circuit capable of providing a more stable output.
According to the present invention, an automatic gain control circuit for amplifying an analog input signal containing an input reference signal to supply a digital output signal is provided with an analog variable gain amplifying circuit, an analog-digital converter, and a digital AGC/detection circuit. The analog variable gain amplifying circuit receives the analog input signal. The analog-digital converter converts the analog signal output from the analog variable gain amplifying circuit into a digital signal. The digital AGC/detection circuit supplies to the analog variable gain amplifying circuit a control signal for controlling the gain of the analog variable gain amplifying circuit such that the level of an input reference signal in the digital signal output from the analog-digital converter remains stable, and amplifies the digital signal to obtain the digital output signal such that the input reference signal in the digital signal becomes equal in level to a prescribed digital reference signal.
Since the above-described AGC/detection circuit is formed by a digital circuit, the level of the input reference signal can be accurately detected even when the ambient temperature or the power supply voltage varies so that a more stable digital output signal is obtained. Moreover, because the digital AGC/detection circuit roughly performs an automatic gain control in advance by feeding back a control signal to the analog variable gain amplifying circuit, a broader range of the analog input signal which keeps the digital output signal stable is realized.
The above-described automatic gain control circuit, preferably, is further provided with a high-cut filter connected between an output of the analog variable gain amplifying circuit and an input of the digital AGC/detection circuit.
Thus, the high-frequency noise component contained in the output signal from the analog variable gain amplifying circuit is removed, and consequently, the level of the input reference signal is more accurately detected.
The above-described high-cut filter, more preferably, is connected between an output of the analog-digital converter and an input of the digital AGC/detection circuit.
Being formed by a digital circuit, the high-cut filter is not affected by the variation in the ambient temperature or in the power supply voltage, and is also capable of removing the clock noise from the analog-digital converter.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.